Gallium nitride-based compound semiconductor multilayer structure and production method thereof

ABSTRACT

An object of the present invention is to provide a gallium nitride compound semiconductor multilayer structure useful for producing a gallium nitride compound semiconductor light-emitting device which operates at low voltage while maintaining a satisfactory light emission output. The inventive gallium nitride compound semiconductor multilayer structure comprises a substrate, and an n-type layer, an active layer, and a p-type layer formed on the substrate, the active layer being sandwiched by the n-type layer and the p-type layer, and the active layer comprising a thick portion and a thin portion, wherein the active layer has a flat lower surface (on the substrate side) and an uneven upper surface so as to form the thick portion and the thin portion.

CROSS REFERENCE TO RELATED APPLICATION

This application is an application filed under 35 U.S.C. §111(a)claiming benefit, pursuant to 35 U.S.C. §119(e)(1), of the filing dateof the Provisional Application No. 60/541,069 filed on Feb. 3, 2004,pursuant to 35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to a gallium nitride compoundsemiconductor multilayer structure useful for producing a high-powerlight-emitting device which emits ultraviolet to blue light, or greenlight, and to a method for producing the semiconductor multilayerstructure.

BACKGROUND ART

In recent years, gallium nitride compound semiconductors have becomeinteresting as materials for producing light-emitting devices which emitlight of short wavelength. Generally, a gallium nitride compoundsemiconductor is grown on a substrate made of an oxide crystal such as asapphire single crystal, a silicon carbide single crystal, or a GroupIII-V compound single crystal, through a method such as metal-organicchemical vapor deposition (MOCVD), molecular-beam epitaxy (MBE), orhydride vapor phase epitaxy (HVPE).

At present, the crystal growth method that is most widely employed inthe industry includes growing a semiconductor crystal on a substratesuch as sapphire, SiC, GaN, or AlN, through metal-organic chemical vapordeposition (MOCVD). Specifically, an n-type layer, an active layer, anda p-type layer are grown on the aforementioned substrate placed in areactor tube, by use of a Group III organometallic compound and a GroupV source gas at about 700° C. to about 1,200° C.

After growth of the above layers, a negative electrode is formed on thesubstrate or the n-type layer, and a positive electrode is formed on thep-type layer, whereby a light-emitting device is fabricated.

Conventionally, such an active layer is formed from InGaN whosecomposition is controlled so as to modulate the light emissionwavelength. The active layer is sandwiched by layers having a bandgaphigher than that of InGaN, thereby forming a double-hetero structure, oris incorporated into a multiple quantum well structure on the basis ofthe quantum well effect.

In a gallium nitride compound semiconductor light-emitting device havingactive layers included in a multiple quantum well structure, when thethickness of a well layer is adjusted to 20 to 30 Å, satisfactory outputis attained, but a problematically high operation voltage is required.In contrast, when the thickness of the well layer is 20 Å or less,operation voltage is lowered, but output is poor.

There has been also proposed a quantum dot structure in which an activelayer in the form of a dot pattern is formed as described below.

For example, Japanese Patent Application Laid-Open (kokai) Nos. 10-79501and 11-354839 disclose light-emitting devices having an active layer ofa quantum dot structure. The quantum dot structure is formed through ananti-surfactant effect. However, the above-proposed quantum dotstructure has a problem. That is, since the total area of dots(light-emitting dots) is excessively small with respect to the areawhere current flows, overall emission output with respect to inputcurrent is lowered, even though the emission efficiency of eachlight-emitting dot is enhanced. These patent documents do not stipulatethe area covered with dots. However, the area that is not covered withdots is considerably greater than the area covered with dots, ascalculated from the dot size and preferred dot density described in thespecifications.

In addition, there has been proposed a quantum box structure including alight-emitting box having an area greater than that of a light-emittingdot.

For example, Japanese Patent Application Laid-Open (kokai) No.2001-68733 discloses an In-containing quantum box structure which isformed by annealing a formed quantum well structure in hydrogen so as tosublimate the well layer. The dimensions of each light-emitting box arepreferably as follows: 0.5 nm≦height≦50 nm and 0.5 nm≦width≦200 nm, anda light-emitting box (height: 6 nm, width: 40 nm) is fabricated in aWorking Example. Although the light-emitting box density is notstipulated, the area which is not covered with light-emitting boxes isgreater than or equal to the area which is covered with light-emittingboxes, as shown in an attached drawing.

Briefly, each of the structures based on the aforementioned techniquesdo not include quantum dots or quantum boxes in the area on whichquantum dots or boxes are not provided. In addition, the area which iscovered with quantum boxes or dots is very small and, in contrast, thearea which is not covered with quantum boxes or dots is larger.

In such a structure in which the area that is covered withlight-emitting boxes or dots is very small and no light-emittingelements are provided in the area that is not covered with quantum boxesor dots, the operation voltage can be lowered, but emission output isproblematically reduced. Thus, such a structure cannot be used inpractice.

Japanese Patent Application Laid-Open (kokai) No. 2001-68733 alsodiscloses that a quantum box structure is fabricated by forming aconventional quantum well structure and annealing the structure inhydrogen, thereby decomposing an InGaN crystal provided on through-holedislocations. However, annealing a quantum well structure in hydrogeninduces a release of In from a portion to serve as a quantum boxstructure, thereby blue-shifting the emission wavelength, which is notpreferred.

Also, in US Patent Application Publication No. US2003/0160229A1, aquantum well structure, in which a well layer has a thickness whichchanges periodically, is disclosed. The well layer has depressions andprotrusions in upper and lower surfaces, which means that an uppersurface of a barrier layer, which fills up the depressions of the welllayer, is not flat. In such structure, although the operation voltagecan be lowered, emission output is reduced.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a gallium nitridecompound semiconductor multilayer structure useful for producing agallium nitride compound semiconductor light-emitting device whichoperates at low voltage while maintaining satisfactory light emissionoutput.

Another object of the invention is to provide a method for forming anactive layer which prevents blue-shifting of emitted light.

The present invention provides the following.

(1) A gallium nitride compound semiconductor multilayer structurecomprising a substrate, and an n-type layer, an active layer, and ap-type layer formed on the substrate, the active layer being sandwichedby the n-type layer and the p-type layer, and the active layercomprising a thick portion and a thin portion, wherein the active layerhas a flat lower surface (on the substrate side) and an uneven uppersurface so as to form the thick portion and the thin portion.

(2) A gallium nitride compound semiconductor multilayer structureaccording to (1) above, wherein the active layer contains In.

(3) A gallium nitride compound semiconductor multilayer structureaccording to (2) above, wherein the upper surface of the active layer iscovered with a thin layer not containing In.

(4) A gallium nitride compound semiconductor multilayer structureaccording to any one of (1) to (3) above, wherein the thick portion hasa thickness of 15 Å to 50 Å.

(5) A gallium nitride compound semiconductor multilayer structureaccording to (4) above, wherein the thick portion has a thickness of 15Å to 30 Å.

(6) A gallium nitride compound semiconductor multilayer structureaccording to any one of (1) to (5) above, wherein the thick portion hasan arithmetic mean width, as measured in a cross-section of themultilayer structure, of 10 nm or more.

(7) A gallium nitride compound semiconductor multilayer structureaccording to (6) above, wherein the thick portion has a width, asmeasured in a cross-section of the multilayer structure, of 100 nm ormore.

(8) A gallium nitride compound semiconductor multilayer structureaccording to any one of (1) to (7) above, wherein the thin portion has athickness of 15 Å or less.

(9) A gallium nitride compound semiconductor multilayer structureaccording to any one of (1) to (8) above, wherein the thin portion hasan arithmetic mean width, as measured in a cross-section of themultilayer structure, of 100 nm or less.

(10) A gallium nitride compound semiconductor multilayer structureaccording to (9) above, wherein the thin portion has a width, asmeasured in a cross-section of the multilayer structure, of 50 nm orless.

(11) A gallium nitride compound semiconductor multilayer structureaccording to any one of (1) to (10) above, wherein the difference inthickness between the thick portion and the thin portion falls within arange of 10 Å to 30 Å.

(12) A gallium nitride compound semiconductor multilayer structureaccording to any one of (1) to (11) above, wherein the thick portion hasan area accounting for 30% or more the entire area of the active layer.

(13) A gallium nitride compound semiconductor multilayer structureaccording to (12) above, wherein the thick portion has an areaaccounting for 50% or more the entire area of the active layer.

(14) A gallium nitride compound semiconductor multilayer structureaccording to any one of (1) to (13) above, wherein the active layer isat least one well layer in a multiple quantum well structure.

(15) A gallium nitride compound semiconductor multilayer structureaccording to (14) above, wherein the multiple quantum well structure isrepeatedly stacked 3 to 10 times.

(16) A gallium nitride compound semiconductor multilayer structureaccording to (15) above, wherein the multiple quantum well structure isrepeatedly stacked 3 to 6 times.

(17) A gallium nitride compound semiconductor multilayer structureaccording to any one of (14) to (16) above, wherein the multiple quantumwell structure has a barrier layer formed of a gallium nitride compoundsemiconductor selected from GaN, AlGaN, and InGaN which has an Incontent lower than that of the InGaN forming the active layer.

(18) A gallium nitride compound semiconductor multilayer structureaccording to (17) above, wherein the barrier layer is formed of GaN.

(19) A gallium nitride compound semiconductor multilayer structureaccording to (17) or (18) above, wherein the barrier layer has athickness of 70 Å to 500 Å.

(20) A gallium nitride compound semiconductor multilayer structureaccording to (19) above, wherein the barrier layer has a thickness of160 Å or more.

(21) A gallium nitride compound semiconductor light-emitting device,wherein the device has a negative electrode and a positive electrode,the negative electrode and the positive electrode being provided on then-type layer and the p-type layer of a gallium nitride compoundsemiconductor multilayer structure according to any one of (1) to (20)above, respectively.

(22) A gallium nitride compound semiconductor light-emitting deviceaccording to (21) above, which has a flip-chip-type device structure.

(23) A gallium nitride compound semiconductor light-emitting deviceaccording to (22) above, wherein the positive electrode has areflection-type structure.

(24) A method for producing a gallium nitride compound semiconductormultilayer structure including a substrate, and an n-type layer, anactive layer, and a p-type layer formed on the substrate, the activelayer being sandwiched by the n-type layer and the p-type layer andcomprising a thick portion and a thin portion, wherein the methodcomprises a step of forming the active layer, which step includes a stepof growing a gallium nitride compound semiconductor and a step ofdecomposing or sublimating a portion of the gallium nitride compoundsemiconductor.

(25) A method for producing a gallium nitride compound semiconductormultilayer structure as described in (24) above, wherein the activelayer contains In.

(26) A method for producing a gallium nitride compound semiconductormultilayer structure according to (25) above, wherein the active layeris grown by continuously supplying a nitrogen source and a Group IIImetal source containing In and Ga and, subsequently, a thin layer notcontaining In is formed on a surface of the active layer by stopping thefeeding of the In metal source.

(27) A method for producing a gallium nitride compound semiconductormultilayer structure according to any one of (24) to (26) above, whereinthe step of growing is performed at a substrate temperature of T1 andthe step of decomposing or sublimating is performed at a substratetemperature of T2, wherein T1 and T2 satisfy the relationship: T1≦T2.

(28) A method for producing a gallium nitride compound semiconductormultilayer structure according to (27) above, wherein T1 falls within arange of 650 to 900° C.

(29) A method for producing a gallium nitride compound semiconductormultilayer structure according to (28) above, wherein T2 falls within arange of 700 to 1,000° C.

(30) A method for producing a gallium nitride compound semiconductormultilayer structure according to any one of (24) to (29) above, whereinthe step of growing is performed in an atmosphere containing a nitrogensource and a Group III metal source and the step of decomposing orsublimating is performed in an atmosphere containing a nitrogen sourcebut not containing a Group III metal source.

(31) A method for producing a gallium nitride compound semiconductormultilayer structure according to (30) above, wherein the step ofdecomposing or sublimating is performed while the substrate temperatureT1 is elevated to T2.

(32) A method for producing a gallium nitride compound semiconductormultilayer structure according to (31) above, wherein the substratetemperature T1 is elevated to T2 at a temperature elevation rate of 1°C./min to 100° C./min.

(33) A method for producing a gallium nitride compound semiconductormultilayer structure according to (32) above, wherein the temperatureelevation rate is 5° C./min to 50° C./min.

(34) A method for producing a gallium nitride compound semiconductormultilayer structure according to any one of (31) to (33) above, whereinthe substrate temperature T1 is elevated to T2 over 30 seconds to 10minutes.

(35) A method for producing a gallium nitride compound semiconductormultilayer structure according to (34) above, wherein the substratetemperature T1 is elevated to T2 over one minute to five minutes.

(36) A method for producing a gallium nitride compound semiconductormultilayer structure according to any one of (27) to (35) above, whereinthe active layer is at least one well layer in a multiple quantum wellstructure, and at least one barrier layer in the multiple quantum wellstructure is grown at T2, followed by lowering the substrate temperatureto T3 at which further growth is performed.

(37) A method for producing a gallium nitride compound semiconductormultilayer structure according to (36) above, wherein T3 is equal to T1.

According to the gist of the present invention; i.e., the active layerhaving a flat lower surface (on the substrate side) and an uneven uppersurface so as to form the thick portion and the thin portion, a galliumnitride compound semiconductor light-emitting device which maintainssatisfactory output and exhibits a reduced operation voltage can beproduced.

Through formation of the thin portion of the active layer in thepresence of a nitrogen source, blue-shifting of the light emitted fromthe active layer can be prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an exemplary cross-section TEM photograph of the galliumnitride compound semiconductor multilayer structure fabricated inExample 1.

FIG. 2 is another exemplary cross-section TEM photograph of the galliumnitride compound semiconductor multilayer structure fabricated inExample 1.

FIG. 3 is a schematic view of a cross-section of the gallium nitridecompound semiconductor multilayer structure fabricated in Example 1.

FIG. 4 is a schematic view of an electrode configuration of thelight-emitting diode fabricated in Example 1 and 2.

FIG. 5 is an exemplary cross-section TEM photograph of the galliumnitride compound semiconductor multilayer structure fabricated inComparative Example 1.

FIG. 6 is another exemplary cross-section TEM photograph of the galliumnitride compound semiconductor multilayer structure fabricated inComparative Example 1.

BEST MODES FOR CARRYING OUT THE INVENTION

The n-type layer, active layer, and p-type layer of a gallium nitridecompound semiconductor light-emitting device are widely known to beformed from a variety of gallium nitride compound semiconductorsrepresented by formula: Al_(x)In_(y)Ga_(1-x-y)N (0≦x<1; 0≦y<1; 0≦x+y<1).No particular limitation is imposed on the gallium nitride compoundsemiconductor for forming the n-type layer, active layer, and p-typelayer employed in the present invention, and a variety of galliumnitride compound semiconductors represented by formula:Al_(x)In_(y)Ga_(1-x-y) (0≦x<1; 0≦y<1; 0≦x+y<1) may also be employed.

No particular limitation is imposed on the type of the substrate, andthere may be employed conventionally known substrate species such assapphire, SiC, GaP, GaAs, Si, ZnO, and GaN.

In order to form a gallium nitride compound semiconductor on any of theabove substrates (excepting a GaN substrate) which are not theoreticallylattice-matched with the gallium nitride compound, a low-temperaturebuffer method (disclosed in, for example, Japanese Patent 3026087 andJapanese Patent Application Laid-Open (kokai) No. 4-297023) and alattice-mismatch crystal epitaxial growth technique (disclosed in, forexample, Japanese Patent Application Laid-Open (kokai) No. 2003-243302),which is called “seeding process (SP),” may be employed. Among thesemethods, from the viewpoint of productivity and other factors, the SPmethod is a particularly advantageous lattice-mismatch crystal epitaxialgrowth technique which can produce AlN crystal film at the hightemperature that enables formation of GaN crystals.

When a lattice-mismatch crystal epitaxial growth technique such as alow-temperature buffer method or an SP method is employed, the galliumnitride compound semiconductor, which is formed on the buffer layer asan undercoat layer, is preferably GaN which is undoped or lightly doped(dopant concentration is about 5×10¹⁷ cm⁻³). The undercoat layerpreferably has a thickness of 1 to 20 μm, more preferably 5 to 15 μm.

In the present invention, the active layer is formed from a thickportion and a thin portion. As used herein, the term “thick portion”refers to a portion having a thickness not less than an averagethickness of the active layer, and the term “thin portion” refers to aportion having a thickness less than an average thickness of the activelayer. The term “average thickness” is the arithmetic mean of thelargest thickness and the smallest thickness. When the thin portionincludes an area that is not covered with an active layer or has a verythin active layer, the thick portion refers to a portion having athickness not less than ½ the largest thickness of the active layer, andthe thin portion refers to a portion having a thickness less than ½ thelargest thickness of the active layer.

The thick portion and the thin portion can be determined visually andquantitatively from a TEM cross-section photograph of a gallium nitridecompound semiconductor. For example, from a TEM cross-section photographof the compound semiconductor at a magnification of 500,000 to2,000,000, the thickness and width of the thick portion or the thinportion can be determined. FIG. 1 shows a TEM cross-section photographof a semiconductor sample fabricated in Example 1 at a magnification of2,000,000. In FIG. 1, reference numeral 1 denotes an active layer (welllayer), and each of A, B, and C denotes a thin portion. Referencenumerals 2, 3 and 4 denote a barrier layer, n-type cladding layer andp-type cladding layer, respectively. The width and thickness of a thickportion or a thin portion can be calculated by use of the magnification.FIG. 2 shows a TEM cross-section photograph of the same sample 1 at amagnification of 500,000. In FIG. 2, reference numeral 1 denotes anactive layer (well layer), and each of D, E, F, and G denotes a thinportion. Reference numerals 2, 3 and 4 denote a barrier layer, n-typecladding layer and p-type cladding layer, respectively. The width andthickness of a thick portion or a thin portion can be calculated by useof the magnification.

The thickness or the width of a thick portion or a thin portion is anarithmetically averaged value obtained in a plurality of observationfields for measurement in a TEM cross-section photograph (e.g., observedin 10 fields, adjacent fields being spaced at a pitch of 10 μm).

The active layer preferably has a virtually flat lower surface (on thesubstrate side) and an uneven upper surface, which forms depressions andprotrusions, so as to form the thick portion and the thin portion.Through employment of such a structure, a drop in emission intensity anddeterioration due to aging can be prevented.

As used herein, the expression “flat” refers to the case where adifference of height between a depressed portion and a protrudedportion, which is observed from aforementioned TEM cross-sectionphotograph, is, for example, 1 nm or less. It is preferable that thedifference is 0.5 nm or less, and it is more preferable that depressionsand protrusions are scarcely visible.

Also, when the difference at the lower surface is ⅕ or less in contrastto that at the upper surface, a layer which is adjacent to the substrateside of the active layer (for example, a barrier in case of a multiplequantum well structure) has an excellent crystallinity andcharacteristic is improved. It is more preferable that the difference atthe lower surface is 1/10 or less in contrast to that at the uppersurface. It is most preferable that depressions and protrusions arescarcely visible at the lower surface.

The thick portion preferably has a thickness of about 15 Å to about 50Å. When the thickness of the thick portion falls outside the range,emission output is lowered. More preferably, the thickness is 15 Å to 30Å. The width of the thick portion is preferably 10 to 5,000 nm, morepreferably 100 to 1,000 nm.

The active layer may include a thin portion having a thickness of 0. Inother words, the active layer may include an area that is not coveredwith an active layer. However, such an area is preferably narrow,because the absence of an active layer causes lowering of emissionoutput. Thus, the area preferably accounts for 30% or less the entirearea of the active layer, more preferably 20% or less, particularlypreferably 10% or less.

The thin portion has a width of 1 to 100 nm, more preferably 5 to 50 nm.

The difference in thickness between the thick portion and the thinportion preferably falls within a range of about 10 to about 30 Å. Thethickness of the thin portion is preferably 15 Å or less.

In the area where current flows, the area that is covered with the thickportion preferably accounts for 30 to 90% the entire active layer. Whenthe area falls within the range, lowering of operation voltage andmaintaining output can be attained. More preferably, the area that iscovered with the thick portion is greater than the area that is coveredwith the thin portion (i.e., accounting for 50% or more the entireactive layer).

The active layer may have a single quantum well structure being formedof a single layer. However, the active layer preferably has a multiplequantum well structure in which a well layer serving as an active layerand a barrier layer are alternatingly stacked repeatedly, from theviewpoint of enhancement of emission output. The repetition of stackingis preferably about 3 to about 10 times, more preferably about 3 toabout 6 times. All the well layers (active layers) included in themultiple quantum well structure do not necessarily have a thick portionand thin portion, and the dimensions and area proportion of each of thethick portions and the thin portions may vary layer by layer. In thespecification, when a multiple quantum well structure is employed, theentirety of the well layers (active layers) in combination with thebarrier layers are referred to as a light-emitting layer.

In a preferred mode, a barrier layer fills up the thin portion of a welllayer, and has a flat upper surface. According to this preferred mode,the lower surface of the subsequently stacked well layer becomes flat.

The barrier layer preferably has a thickness of 70 Å or more, morepreferably 140 Å or more. When the barrier layer is excessively thin,planarization of the upper surface of the barrier layer cannot beattained, leading to lowering of emission efficiency and deteriorationin characteristics due to aging, whereas when the barrier layer isexcessively thick, operation voltage increases and emission becomesweak. Therefore, the barrier layer preferably has a thickness of 500 Åor less.

The active layer is preferably formed of a gallium nitride compoundsemiconductor containing In, as the In-containing gallium nitridecompound semiconductor is of a crystal system for readily attaining astructure having a thick portion and a thin portion through thebelow-mentioned method. In addition, the In-containing gallium nitridecompound semiconductor can emit high-intensity light in a blue lightwavelength region.

When the active layer is formed of an In-containing gallium nitridecompound semiconductor, the upper surface of the active layer ispreferably covered with a thin layer containing no In. By virtue of thethin layer, decomposition/sublimation of In contained in the activelayer can be suppressed, whereby emission wavelength can be consistentlycontrolled, which is preferred.

The active layer may be doped with an impurity element. The dopant ispreferably Si or Ge, which are known to serve as donors, for the purposeof enhancement of emission intensity. The dopant concentration ispreferably about 1×10¹⁷ cm⁻³ to about 1×10¹⁸ cm⁻³. When the amount is inexcess of the upper limit of the range, emission intensity decreases.

In the case where the multiple quantum well structure is employed, thebarrier layer may be formed of GaN, AlGaN, and InGaN which has an Incontent lower than that of InGaN forming a well layer (active layer).Among them, GaN is preferred.

The n-type layer generally has a thickness of about 1 to about 10 μm,preferably about 2 to about 5 μm. The n-type layer is formed of ann-type contact layer for forming a negative electrode and an n-typecladding layer which has a bandgap larger than that of an active layerand which is in contact with the active layer. The n-type contact layermay also serve as the n-type cladding layer. The n-type contact layer ispreferably doped with Si or Ge at high concentration. The thus-dopedn-type layer preferably has a carrier concentration which is controlledto about 5×10¹⁸ cm⁻³ to about 2×10¹⁹ cm⁻³.

The n-type cladding layer may be formed from a semiconductor such asAlGaN, GaN, or InGaN. Needless to say, when InGaN is employed, the InGaNpreferably has such a composition as to have a bandgap greater than thatof InGaN forming the active layer. The carrier concentration of then-type cladding layer may be equal to or greater or smaller than that ofthe n-type contact layer. The n-type cladding layer preferably has asurface having high flatness by appropriately regulating growthconditions including growth rate, growth temperature, growth pressure,and dopant concentration, so as to attain high crystallinity of theactive layer formed thereon.

The n-type cladding layer may be formed by alternatingly stacking layersrepeatedly, each layer having a specific composition and latticeconstants. In this case, in addition to the composition, the amount ofdopant, film thickness, etc. of the layer stacked may be modified.

The p-type layer generally has a thickness of 0.01 to 1 μm and is formedof a p-type cladding layer which is in contact with the active layer anda p-type contact layer for forming a positive electrode. The p-typecladding layer may also serve as the p-type contact layer. The p-typecladding layer is formed from a semiconductor such as GaN or AlGaN anddoped with Mg serving as a p-type dopant. In order to prevent overflowof electrons, the p-type cladding layer is preferably formed from amaterial having a bandgap greater than that of the material for formingthe active layer. Furthermore, in order to effectively inject carries tothe active layer, the p-type cladding layer preferably has high carrierconcentration.

Similar to the n-type cladding layer, the p-type cladding layer may beformed by alternatingly stacking layers repeatedly, each layer having aspecific composition and lattice constants. In this case, in addition tothe composition, the amount of dopant, film thickness, etc. of thestacked layer may be modified.

The p-type contact layer may be formed from a semiconductor such as GaN,AlGaN, or InGaN, and is doped with Mg serving as an impurity element.When removed from a reactor, the as-removed Mg-doped gallium nitridesemiconductor compound semiconductor generally exhibits high electricresistance. However, the Mg-doped compound semiconductor exhibits p-typeconductivity through activation such as annealing, electron-beamirradiation, or microwave irradiation.

The p-type contact layer may be formed from boron phosphide doped with ap-type impurity element, which exhibits p-type conductivity even thoughthe aforementioned treatment for attaining p-type conductivity is notperformed.

No particular limitation is imposed on the method for growing thegallium nitride compound semiconductor for forming the aforementionedn-type layer, active layer, and p-type layer, and any of widely knownmethods such as MBE, MOCVD, and HVPE may be employed under widely knownconditions. Of these, MOCVD is preferred.

Regarding sources for forming the semiconductor, ammonia, hydrazine, anazide, or a similar compound may be used as a nitrogen source. Examplesof the Group III organometallic source which may be employed in theinvention include trimethylgallium (TMGa), triethylgallium (TEGa),trimethylindium (TMIn), and trimethylaluminum (TMAl). Examples ofemployable dopant sources include silane, disilane, germane, organicgermanium sources, and biscyclopentadienylmagnesium (Cp₂Mg). Nitrogen orhydrogen may be employed as a carrier gas.

Preferably, the active layer including a thick potion and a thin portionis formed by growing a gallium nitride compound semiconductor anddecomposing or sublimating a portion of the grown semiconductor. AnIn-containing gallium nitride compound semiconductor is preferred,because the semiconductor is readily decomposed or sublimated.

The In-containing active layer is preferably grown at a substratetemperature of 650 to 900° C. When the substrate temperature is lowerthan 650° C., an active layer of high crystallinity cannot be formed,whereas when the substrate temperature is higher than 900° C., theamount of In incorporated into the active layer decreases, therebyfailing to fabricating a light-emitting device which emits light ofintended wavelength.

As described above, when the active layer contains In, a thin layercontaining no In is preferably formed on the surface of the activelayer. In this case, after completion of growth of an In-containinggallium nitride compound semiconductor, a gallium nitride compoundsemiconductor is grown at the same substrate temperature while thesupply of the In source is stopped.

After the In-containing gallium nitride compound semiconductor has beengrown to a predetermined thickness by continuously supplying a Group IIImetal (containing In) source and a nitrogen source, supply of the GroupIII metal source is stopped. The substrate temperature is maintained orelevated under the above conditions, thereby decomposing or sublimatinga portion of the compound semiconductor. The carrier gas is preferablynitrogen. Decomposition or sublimation is preferably performed, when thesubstrate temperature has been elevated from the above growthtemperature to 700 to 1,000° C. or while the substrate temperature iselevated.

In the case where the active layer has a multiple quantum wellstructure, the barrier layer is preferably grown at a substratetemperature higher than that employed for the growth of the well layer(active layer). The substrate temperature is preferably 700 to 1,000° C.

When the temperature at which the well layer is grown is represented byT1 and the temperature at which the barrier layer is grown isrepresented by T2, T1 and T2 satisfy the relationship: T1≦T2. Duringtemperature elevation after growth of the well layer from T1 to T2,supply of the Group III source is stopped while the nitrogen source anda nitrogen-containing carrier gas are supplied continuously, whereby athick portion and a thin portion are effectively formed in the welllayer. During the course of the above procedure, a change of the carriergas is not needed. If the carrier gas is changed to hydrogen, thewavelength of emitted light is blue-shifted. As such variation inwavelength is difficult to control reliably, the variation reduces thedevice productivity.

The rate of temperature elevation from T1 to T2 is preferably about 1 toabout 100° C./min, more preferably about 5 to about 50° C./min. The timerequired for temperature elevation from T1 to T2 is preferably about 30sec to about 10 min, more preferably about 1 min to about 5 min.

The growth of the barrier layer may include a plurality of steps whichare performed at different growth temperatures. For example, a barrierlayer having a predetermined thickness is formed at T2 on a well layerhaving a thick portion and a thin portion, followed by forming thereonanother barrier layer at a growth temperature T3. When T3 is lower thanT2, deterioration of characteristics caused by aging can be prevented,which is preferred. T3 may be equal to T1.

Negative electrodes of a variety of compositions and structures havebeen widely known, and no particular limitation is imposed on thenegative electrode which may be employed in the present invention.Examples of employable contact materials for the negative electrodewhich is to be in contact with an n-type contact layer include Al, Ti,Ni, Au, Cr, W, and V. Needless to say, the negative electrode may have amultilayer structure in its entirety, thereby imparting the electrodewith a bonding property and other properties.

Positive electrodes of a variety of compositions and structures havebeen widely known, and no particular limitation is imposed on thepositive electrode which may be employed in the present invention.

Examples of light-permeable positive electrode materials include Pt, Pd,Au, Cr, Ni, Cu, and Co. Through partial oxidation of the positiveelectrode, light permeability is known to be enhanced. Examples ofemployable reflection-type positive electrode materials include theaforementioned materials, Rh, Ag, and Al.

The positive electrode may be formed through a method such as sputteringor vacuum vapor deposition. Particularly when sputtering is employedunder appropriately controlled sputtering conditions, ohmic contact canbe established even though the electrode film is not annealed afterformation of the film, which is preferred.

The light-emitting device may have a flip-chip-type structure includinga reflection-type positive electrode or a face-up-type structureincluding an light-permeable positive electrode or a lattice-like orcomb-like positive electrode.

According to the active layer of the present invention including a thickportion and a thin portion, an interface between the active layer and ap-type layer composed of a material different from that of the activelayer (in the case of multiple quantum well structure, interface betweenthe well layer (active layer) and a barrier layer) in a boundary areabetween the thick portion and the thin portion is slanted to thesubstrate surface. Therefore, the amount of light extracted in thedirection normal to the substrate surface increases. Particularly whenthe light-emitting device has a flip-chip-type structure including areflection-type electrode, emission intensity is further enhanced.

EXAMPLES

The present invention will be described in more detail by way ofexamples, which should not be construed as limiting the invention.

Example 1

FIG. 3 shows a gallium nitride compound semiconductor multilayerstructure for producing a semiconductor light-emitting device whichstructure was fabricated in Example 1. As shown in FIG. 3, an SP layerformed of AlN is stacked on a sapphire substrate having a c-planethrough a lattice-mismatch crystal epitaxial growth method. On the SPlayer, the following layers are sequentially formed: an undoped GaNundercoat layer (thickness: 2 μm); a highly-Si-doped n-GaN contact layer(electron concentration: 1×10¹⁹ cm⁻³, thickness: 2 μm); ann-In_(0.1)Ga_(0.9)N cladding layer (electron concentration: 1×10¹⁸ cm⁻³,thickness: 125 Å); a light-emitting layer of a multiple quantum wellstructure including GaN barrier layers (6 layers, thickness of eachlayer: 160 Å) and well layers (active layers) (5 layers, each layerbeing formed undoped In_(0.2)Ga_(0.8)N layer (thickness: 25 Å) and a GaNlayer (thickness: 0 to 5 Å)); an Mg-doped p-type Al_(0.07)Ga_(0.93)Ncladding layer (thickness: 100 Å); and an Mg-doped p-GaN contact layer(hole concentration: 8×10¹⁷ cm⁻³, thickness: 0.1 μm).

The aforementioned gallium nitride compound semiconductor multilayerstructure was fabricated by means of MOCVD through the followingprocedure.

Firstly, a sapphire substrate was placed in a stainless reactor furnacethat can heat a plurality of substrates by means of a carbon susceptorheated using an induction heater. The susceptor has a mechanism suchthat the susceptor itself is rotatable and rotates the substrates. Thesapphire substrate was placed on the carbon susceptor for heating, theoperation being performed in a nitrogen-substituted glove box. Afterintroduction of the substrate, the reactor furnace was purged withnitrogen.

After passing nitrogen for 8 minutes, the substrate temperature waselevated, over 10 minutes, to 600° C. by means of the induction heater,and the pressure inside the furnace was adjusted to 150 mbar (15 kPa).While the substrate temperature was maintained at 600° C., the substratesurface was thermally cleaned by allowing the substrate to stand for 2minutes under a flow of hydrogen and nitrogen.

After completion of thermal cleaning, a valve of a nitrogen carrier gasfeeding pipe was closed, and only hydrogen was supplied to the reactorfurnace.

After the carrier gas was changed to hydrogen, the substrate temperaturewas elevated to 1,180° C. After confirmation that a constant temperatureof 1,180° C. was attained, a gas containing TMAl vapor was supplied tothe reactor furnace by opening the corresponding valve. The suppliedTMAl was caused to react with N atoms which had been released throughdecomposition of deposits on an inner wall of the reactor furnace,thereby depositing AlN on the sapphire substrate.

After supply of TMAl for 8 minutes and 30 seconds, the valve was closedto stop supply of the gas containing TMAl vapor to the reactor furnace.The conditions were maintained for 4 minutes, whereby the TMAl vaporremaining in the furnace was completely removed. Subsequently, ammoniagas was supplied to the furnace by opening the corresponding valve.

Four minutes after the start of supply of ammonia gas, the susceptortemperature was lowered to 1,040° C. under ammonia flow. During loweringof the susceptor temperature, the flow rate of TMGa was regulated bymeans of a flow controller.

After confirmation that the susceptor temperature was lowered to 1,040°C. and the susceptor maintained a constant temperature of 1,040° C.,TMGa was supplied into the furnace by opening the corresponding valve,so as to grow undoped GaN. The growth was performed for about one hour,thereby forming the aforementioned GaN layer.

Thus, an undoped GaN undercoat layer having a thickness of 2 μm wasformed.

On the undoped GaN undercoat layer, a highly-Si-doped n-type GaN layerwas grown. After completion of growth of the undoped GaN undercoatlayer, the supply of TMGa into the furnace was interrupted for oneminute, and the flow rate of SiH₄ was adjusted during the interruptionof flow. The flow rate of interest had been predetermined in advance,and the flow was regulated so as to control the electron concentrationof the highly-Si-doped GaN layer to 1×10¹⁹ cm⁻³. Ammonia was suppliedcontinuously into the furnace, but the flow rate was unchanged. Duringthe above interruption of TMGa supply for one minute, the susceptortemperature was elevated from 1,040° C. to 1,060° C.

After the interruption of TMGa supply for one minute, TMGa and SiH₄ weresupplied, and the growth was performed for one hour, thereby forming ahighly-Si-doped n-GaN contact layer having a thickness of 2 μm.

After growth of the highly-Si-doped n-GaN contact layer, supply of TMGaand SiH₄ into the furnace was stopped by closing the correspondingvalves. The carrier gas was changed from hydrogen to nitrogen throughvalve operation, while ammonia was supplied continuously. Thereafter,the substrate temperature was lowered from 1,060° C. to 730° C.

During the lowering of the temperature inside the furnace, the flow rateof SiH₄ was modified. The flow rate of interest had been predeterminedin advance, and the flow was regulated so as to control the electronconcentration of the Si-doped n-InGaN cladding layer to 1×10¹⁸ cm⁻³.Ammonia was supplied continuously into the furnace, but the flow ratewas unchanged.

Subsequently, after the conditions in the furnace had been stabilized,TMIn, TEGa, and SiH₄ were supplied to the furnace by simultaneouslyopening the corresponding valves. The supply was continued for apredetermined period of time, thereby forming an Si-dopedn-In_(0.1)Ga_(0.9)N cladding layer having a thickness of 125 Å. Supplyof the sources (TMIn, TEGa, and SiH₄) was stopped by closing thecorresponding valves.

After completion of growth of the Si-doped n-In_(0.1)Ga_(0.9)N claddinglayer, the susceptor temperature was elevated to 930° C. and thesusceptor was held at a constant temperature of 930° C., TEGa wassupplied to the furnace by opening the corresponding valve whilesubstrate temperature, pressure inside the furnace, flow rate of ammoniaand carrier gas, and the type of carrier gas were maintained constant.The growth was performed for a predetermined period of time at asusceptor temperature of 930° C. Subsequently, the susceptor temperaturewas lowered to 730° C., and TEGa was supplied so as to perform growth.The supply of TEGa was stopped by closing the corresponding valve,thereby terminating the growth of a GaN barrier layer. As a result, aGaN barrier layer having a total thickness of 160 Å was formed.

After completion of growth of the GaN barrier layer, supply of the GroupIII source was stopped for 30 seconds. Subsequently, TEGa and TMIn weresupplied to the furnace by opening the corresponding valves whilesubstrate temperature, pressure inside the furnace, flow rate of ammoniaand carrier gas, and the type of carrier gas were maintained constant.TEGa and TMIn were supplied for a predetermined period of time, andsupply of TMIn was stopped by closing the corresponding valve, therebyterminating the growth of an In_(0.2)Ga_(0.8)N well layer (activelayer). In this instance, an In_(0.2)Ga_(0.8)N layer having a thicknessof 25 Å was formed.

After completion of the growth of the In_(0.2)Ga_(0.8)N layer, only TEGawas supplied continuously to the furnace for a predetermined period oftime, thereby forming, on the InGaN layer, a GaN thin layer (cappinglayer) for preventing the release of In. Then, supply of TEGa wasstopped.

Subsequently, while N₂ serving as a carrier gas and NH₃ were suppliedcontinuously, the susceptor temperature was elevated to 930° C. over twominutes. Through this operation, a portion of the In_(0.2)Ga_(0.8)Nlayer was decomposed and sublimated, thereby removing a portion of theIn_(0.2)Ga_(0.8)N layer and reducing the thickness of the layer. Thus, athin portion having a small thickness was formed in the well layer(active layer).

The aforementioned procedure was repeated five times, to thereby formfive GaN barrier layers and five In_(0.2)Ga_(0.8)N well layers. Finally,another GaN barrier was formed, to thereby fabricate a light-emittinglayer having a multiple quantum well structure.

On the outermost GaN barrier layer of the light-emitting layer, anMg-doped p-type Al_(0.07)Ga_(0.93)N cladding layer was formed.

After completion of the growth of the last GaN barrier layer by stoppingthe supply of TEGa, the substrate temperature was elevated to 1,020° C.The carrier gas was changed to hydrogen, and the pressure inside thefurnace was adjusted to 150 mbar (15 kPa). After the pressure inside thefurnace became constant, sources (TMGa, TMAl, and Cp₂Mg) were suppliedto the furnace by opening the corresponding valves. The growth wasperformed for about three minutes, after which supply of TEGa and TMAlwas stopped, thereby terminating the growth of an Mg-doped p-typeAl_(0.07)Ga_(0.93)N cladding layer. As a result, an Mg-doped p-typeAl_(0.07)Ga_(0.93)N cladding layer having a thickness of 100 Å wasformed.

On the Mg-doped p-type Al_(0.07)Ga_(0.93)N cladding layer, an Mg-dopedp-type GaN contact layer was formed.

After completion of the growth of the Mg-doped p-Al_(0.07)Ga_(0.93)Ncladding layer by stopping the supply of TMGa, TMAl, and Cp₂Mg, thepressure inside the furnace was adjusted to 200 mbar (20 kPa). After thepressure inside the furnace became constant, sources (TMGa and Cp₂Mg)were supplied to the furnace by opening the corresponding valves. Theflow rate of Cp₂Mg had been predetermined in advance, and the flow wasregulated so as to control the hole concentration of the Mg-doped p-GaNcontact layer to 8×10¹⁷ cm⁻³. Thereafter, the growth was performed forabout four minutes, after which supply of TMGa and Cp₂Mg was stopped,thereby terminating the growth of the Mg-doped GaN layer. As a result,the Mg-doped p-GaN contact layer was formed to a thickness of 0.1 μm.

After completion of the growth of the Mg-doped p-GaN contact layer, theelectricity supply to the induction heater was stopped, and thesubstrate temperature was lowered to room temperature over 20 minutes.During the process of lowering the temperature, the atmosphere in thereactor furnace was exclusively nitrogen. When the substrate temperaturewas confirmed to have been lowered to room temperature, thethus-fabricated gallium nitride compound semiconductor multilayerstructure was removed to the atmosphere.

Through the above-described procedure, the gallium nitride compoundsemiconductor multilayer structure for producing a semiconductorlight-emitting device was fabricated. Even though the Mg-doped GaN layerhad not undergone annealing for activating the p-type carrier, the GaNlayer exhibited p-type conductivity.

By use of the aforementioned gallium nitride compound semiconductormultilayer structure, a light-emitting diode, a type of semiconductorlight-emitting device, was fabricated.

On the surface of the p-type GaN contact layer of the thus-fabricatedgallium nitride compound semiconductor multilayer structure, there wasformed a reflection-type positive electrode having a structure in whichPt, Rh, and Au were successively formed on the contact layer sidethrough a conventional photolithographic method.

Subsequently, the aforementioned gallium nitride compound semiconductormultilayer structure was dry-etched so as to expose a negative electrodeportion of the highly-Si-doped n-type GaN contact layer. Ti and Al weresuccessively formed on the exposed portion of the contact layer, therebyforming a negative electrode. Through these operations, electrodes ofthe shape shown in FIG. 4 were fabricated.

The back of the sapphire substrate of the gallium nitride compoundsemiconductor multilayer structure which had been provided with thepositive electrode and the negative electrode in the above manner wasground and polished, thereby providing a mirror surface. Subsequently,the gallium nitride compound semiconductor multilayer structure was cutinto square (350 μm×350 μm) chips, and each chip was affixed on asub-mount such that the electrodes were in contact with the sub-mount.The thus-formed sub-mounted chip was placed on a lead frame and wired tothe lead frame with gold wire, thereby fabricating a light-emittingdevice.

When an operating current was applied to the positive electrode and thenegative electrode of the thus-fabricated light-emitting diode in aforward direction, the diode exhibited a forward voltage of 3.0 V at acurrent of 20 mA, an emission wavelength of 455 nm, and an emissionoutput of 10 mW. Such characteristics of the light-emitting diode can beattained without variation among light-emitting diodes cut and producedfrom virtually the entirety of the above-fabricated gallium nitridecompound semiconductor multilayer structure.

The thus-fabricated gallium nitride compound semiconductor multilayerstructure was observed under a cross-section TEM, and FIGS. 1 and 2 showphotographs thereof (magnification: 2,000,000 (FIG. 1) and 500,000 (FIG.2)).

As shown in FIGS. 1 and 2, each well layer serving as an active layer isidentified as being formed of a thick portion and a thin portion.

The observed thick portion was found to have a thickness of 25 Å and awidth of 500 Å, and the observed thin portion was found to have a widthof 50 Å and a thickness of 10 Å or less. It was observed that someportions of the well layer were completely removed.

From the TEM cross-section photographs, the area of the thick portionwas found to account for 90% to 60% of the entire area of the activelayer.

The barrier layers were found to have a thickness of 160 Å. Each barrierlayer leveled the surface of each well layer having a thin portion and athick portion, and each well layer was found to have a flat bottomsurface. The difference in thickness between the thick portion and thethin portion is predominantly attributable to depressions andprotrusions formed on the upper surface of each well layer.

Comparative Example 1

In Comparative Example 1, the procedure of Example 1 was repeated,except that a different light-emitting layer was employed, to therebyfabricate a gallium nitride compound semiconductor multilayer structureof the same configuration. The light-emitting layer of ComparativeExample 1 is different from that of Example 1 in that the a well layer(active layer) having a uniform thickness and a barrier layer having auniform thickness were repeatedly stacked.

The procedure of Comparative Example 1 for fabricating a gallium nitridecompound semiconductor multilayer structure was different from that ofExample 1 in the following. Specifically, in Example 1, after formationof a capping layer (GaN thin layer) of the well layer, supply of TEGawas stopped, and the temperature was elevated from 730° C. to 930° C.over two minutes. Then, a barrier layer was formed. However, inComparative Example 1, after formation of a GaN thin layer (cappinglayer) of the well layer, the temperature was elevated from 730° C. to930° C. over two minutes while TEGa was supplied continuously. Then, abarrier layer was formed at 930° C.

In a manner similar to that of Example 1, the light-emitting diode wasfabricated from the gallium nitride compound semiconductor multilayerstructure and was evaluated. As a result, the diode exhibited a forwardvoltage of 3.9 V at a current of 20 mA, an emission wavelength of 455nm, and an emission output of 8.5 mW.

The thus-fabricated gallium nitride compound semiconductor multilayerstructure was observed under a cross-section TEM, and FIGS. 5 and 6 showphotographs thereof (magnification: 2,000,000 (FIG. 5) and 500,000 (FIG.6)). As shown in FIGS. 5 and 6, each well layer (active layer) has avirtually uniform thickness of about 25 Å, and no position-dependentvariation in thickness was found.

Example 2

In this example, a gallium nitride compound semiconductor multilayerstructure was fabricated as follows.

An SP layer formed of AlN is stacked on a sapphire substrate having ac-plane through a lattice-mismatch crystal epitaxial growth method. Onthe SP layer, the following layers are sequentially formed: an undopedGaN undercoat layer (thickness: 8 μm); a n-GaN contact layer in which ahighly-Ge-doped layer and a slightly-Ge-doped layer is alternatelystacked 100 times (average electron concentration: 5×10¹⁸ cm⁻³,thickness: 4 μm); an n-In_(0.1)Ga_(0.9)N cladding layer (electronconcentration: 1×10¹⁸ cm⁻³, thickness: 180 Å); a light-emitting layer ofa multiple quantum well structure including GaN barrier layers (6layers, thickness of each layer: 160 Å) and well layers (active layers)(5 layers, each layer being formed undoped In_(0.2)Ga_(0.8)N layer(thickness: 25 Å) and a GaN layer (thickness: 0 to 5 Å)); an Mg-dopedp-type Al_(0.07)Ga_(0.93)N cladding layer (thickness: 100 Å); and anMg-doped p-GaN contact layer (hole concentration: 8×10¹⁷ cm⁻³,thickness: 0.1 μm).

The aforementioned gallium nitride compound semiconductor multilayerstructure was fabricated by means of MOCVD through the procedure similarto that of Example 1.

Next, by use of the aforementioned gallium nitride compoundsemiconductor multilayer structure, a light-emitting diode, a type ofsemiconductor light-emitting device, was fabricated through thefollowing procedure.

On the surface of the p-type GaN contact layer of the thus-fabricatedgallium nitride compound semiconductor multilayer structure, there wasformed a transparent-type positive electrode having a structure in whichPt and Au were successively formed on the contact layer side, through aconventional photolithographic method. Then, on the positive electrode,there was formed a pad electrode having a structure in which Ti, Au, Aland Au were successively formed on the positive electrode side

Subsequently, the gallium nitride compound semiconductor multilayerstructure was dry-etched so as to expose a negative electrode portion ofthe n-type GaN contact layer. Ti and Al were successively formed on theexposed portion of the contact layer, thereby forming a negativeelectrode. Through these operations, electrodes of a shape shown in FIG.4 were fabricated.

The back of the sapphire substrate of the gallium nitride compoundsemiconductor multilayer structure, which had been provided with thepositive electrode and the negative electrode in the above manner, wasground and polished, thereby providing a mirror surface. Subsequently,the gallium nitride compound semiconductor multilayer structure was cutinto square (350 μm×350 μm) chips. The thus-formed chip was placed on alead frame and wired to the lead frame with gold wire, therebyfabricating a light-emitting device.

When an operation current was applied to the positive electrode and thenegative electrode of the thus-fabricated light-emitting diode in aforward direction, the diode exhibited a forward voltage of 3.2 V at acurrent of 20 mA, an emission wavelength of 470 nm, and an emissionoutput of 6 mW. Such characteristics of the light-emitting diode can beattained without variation among light-emitting diodes cut and producedfrom virtually the entirety of the above-fabricated gallium nitridecompound semiconductor multilayer structure.

Comparative Example 2

In Comparative Example 2, a light-emitting diode having the sameelectrode structure as employed in the diode of Example 2 was fabricatedby use of the gallium nitride compound semiconductor multilayerstructure fabricated in Comparative Example 1.

In a manner similar to that of Example 2, the fabricated light-emittingdiode was evaluated. As a result, the diode exhibited a forward voltageof 3.9 V at a current of 20 mA, an emission wavelength of 455 nm, and anemission output of 5 mW.

INDUSTRIAL APPLICABILITY

The light-emitting device produced from the gallium nitride compoundsemiconductor multilayer structure of the present invention operates atlow voltage while maintaining satisfactory light emission output. Thus,the present invention is of remarkably great value in industry.

1. A gallium nitride compound semiconductor multilayer structurecomprising a substrate, and an n-type layer, an active layer, and ap-type layer formed on the substrate, the active layer being sandwichedby the n-type layer and the p-type layer, and the active layercomprising a thick portion and a thin portion, wherein the active layerhas a flat lower surface (on the substrate side) and an uneven uppersurface so as to form the thick portion and the thin portion.
 2. Agallium nitride compound semiconductor multilayer structure according toclaim 1, wherein the active layer contains In.
 3. A gallium nitridecompound semiconductor multilayer structure according to claim 2,wherein the upper surface of the active layer is covered with a thinlayer not containing In.
 4. A gallium nitride compound semiconductormultilayer structure according to claim 1, wherein the thick portion hasa thickness of 15 Å to 50 Å.
 5. A gallium nitride compound semiconductormultilayer structure according to claim 4, wherein the thick portion hasa thickness of 15 Å to 30 Å.
 6. A gallium nitride compound semiconductormultilayer structure according to claim 1, wherein the thick portion hasan arithmetic mean width, as measured in a cross-section of themultilayer structure, of 10 nm or more.
 7. A gallium nitride compoundsemiconductor multilayer structure according to claim 6, wherein thethick portion has a width, as measured in a cross-section of themultilayer structure, of 100 nm or more.
 8. A gallium nitride compoundsemiconductor multilayer structure according to claim 1, wherein thethin portion has a thickness of 15 Å or less.
 9. A gallium nitridecompound semiconductor multilayer structure according to claim 1,wherein the thin portion has an arithmetic mean width, as measured in across-section of the multilayer structure, of 100 nm or less.
 10. Agallium nitride compound semiconductor multilayer structure according toclaim 9, wherein the thin portion has a width, as measured in across-section of the multilayer structure, of 50 nm or less.
 11. Agallium nitride compound semiconductor multilayer structure according toclaim 1, wherein the difference in thickness between the thick portionand the thin portion falls within a range of 10 Å to 30 Å.
 12. A galliumnitride compound semiconductor multilayer structure according to claim1, wherein the thick portion has an area accounting for 30% or more theentire area of the active layer.
 13. A gallium nitride compoundsemiconductor multilayer structure according to claim 12, wherein thethick portion has an area accounting for 50% or more the entire area ofthe active layer.
 14. A gallium nitride compound semiconductormultilayer structure according to claim 1, wherein the active layer isat least one well layer in a multiple quantum well structure.
 15. Agallium nitride compound semiconductor multilayer structure according toclaim 14, wherein the multiple quantum well structure is repeatedlystacked 3 to 10 times.
 16. A gallium nitride compound semiconductormultilayer structure according to claim 15, wherein the multiple quantumwell structure is repeatedly stacked 3 to 6 times.
 17. A gallium nitridecompound semiconductor multilayer structure according to claim 1,wherein the multiple quantum well structure has a barrier layer formedof a gallium nitride compound semiconductor selected from GaN, AlGaN,and InGaN which has an In content lower than that of the InGaN formingthe active layer.
 18. A gallium nitride compound semiconductormultilayer structure according to claim 17, wherein the barrier layer isformed of GaN.
 19. A gallium nitride compound semiconductor multilayerstructure according to claim 17, wherein the barrier layer has athickness of 70 Å to 500 Å.
 20. A gallium nitride compound semiconductormultilayer structure according to claim 19, wherein the barrier layerhas a thickness of 160 Å or more.
 21. A gallium nitride compoundsemiconductor light-emitting device, wherein the device has a negativeelectrode and a positive electrode, the negative electrode and thepositive electrode being provided on the n-type layer and the p-typelayer of a gallium nitride compound semiconductor multilayer structureaccording to claim 1, respectively.
 22. A gallium nitride compoundsemiconductor light-emitting device according to claim 21, which has aflip-chip-type device structure.
 23. A gallium nitride compoundsemiconductor light-emitting device according to claim 22, wherein thepositive electrode has a reflection-type structure.
 24. A method forproducing a gallium nitride compound semiconductor multilayer structureincluding a substrate, and an n-type layer, an active layer, and ap-type layer formed on the substrate, the active layer being sandwichedby the n-type layer and the p-type layer and comprising a thick portionand a thin portion, wherein the method comprises a step of forming theactive layer, which step includes a step of growing a gallium nitridecompound semiconductor and a step of decomposing or sublimating aportion of the gallium nitride compound semiconductor.
 25. A method forproducing a gallium nitride compound semiconductor multilayer structureaccording to claim 24, wherein the active layer contains In.
 26. Amethod for producing a gallium nitride compound semiconductor multilayerstructure according to claim 25, wherein the active layer is grown bycontinuously supplying a nitrogen source and a Group III metal sourcecontaining In and Ga and, subsequently, a thin layer not containing Inis formed on a surface of the active layer by stopping the feeding ofthe In metal source.
 27. A method for producing a gallium nitridecompound semiconductor multilayer structure according to claim 24,wherein the step of growing is performed at a substrate temperature ofT1 and the step of decomposing or sublimating is performed at asubstrate temperature of T2, wherein T1 and T2 satisfy the relationship:T1≦T2.
 28. A method for producing a gallium nitride compoundsemiconductor multilayer structure according to claim 27, wherein T1falls within a range of 650 to 900° C.
 29. A method for producing agallium nitride compound semiconductor multilayer structure according toclaim 28, wherein T2 falls within a range of 700 to 1,000° C.
 30. Amethod for producing a gallium nitride compound semiconductor multilayerstructure according to claim 24, wherein the step of growing isperformed in an atmosphere containing a nitrogen source and a Group IIImetal source and the step of decomposing or sublimating is performed inan atmosphere containing a nitrogen source but not containing a GroupIII metal source.
 31. A method for producing a gallium nitride compoundsemiconductor multilayer structure according to claim 30, wherein thestep of decomposing or sublimating is performed while the substratetemperature T1 is elevated to T2.
 32. A method for producing a galliumnitride compound semiconductor multilayer structure according to claim31, wherein the substrate temperature T1 is elevated to T2 at atemperature elevation rate of 1° C./min to 100° C./min.
 33. A method forproducing a gallium nitride compound semiconductor multilayer structureaccording to claim 32, wherein the temperature elevation rate is 5°C./min to 50° C./min.
 34. A method for producing a gallium nitridecompound semiconductor multilayer structure according to claim 31,wherein the substrate temperature T1 is elevated to T2 over 30 secondsto 10 minutes.
 35. A method for producing a gallium nitride compoundsemiconductor multilayer structure according to claim 34, wherein thesubstrate temperature T1 is elevated to T2 over one minute to fiveminutes.
 36. A method for producing a gallium nitride compoundsemiconductor multilayer structure according to claim 27, wherein theactive layer is at least one well layer in a multiple quantum wellstructure, and at least one barrier layer in the multiple quantum wellstructure is grown at T2, followed by lowering the substrate temperatureto T3 at which further growth is performed.
 37. A method for producing agallium nitride compound semiconductor multilayer structure according toclaim 36, wherein T3 is equal to T1.